A patterned wafer is a semiconductor wafer with a diameter of several inches and containing a few hundred integrated circuit chips or "die", each resembling an island with a rectangular grid of "streets" between islands. The present invention is a quality control measurement apparatus in which the die on a patterned wafer are inspected for particles and other defects. A particle of dirt will create a risk of failure of the integrated circuit on a die and so a die identified as having a dirt particle thereon must either be cleaned or discarded.
Previously, wafers were most often inspected for particles by operators using conventional optical microscopes. Since this process is time-consuming, labor intensive, and requires that the wafers be handled, particle inspections were performed infrequently, usually as a statistical process control measure after a process was found to be "particle-prone" to implement appropriate changes to the process. Recently, automatic foreign-particle inspection devices have been developed. For example, in a typical apparatus, a laser beam is shone vertically onto a sample and the light scattered by the foreign particles is detected by a photodetector placed at an angle above the sample. Unfortunately, if the sample has surface patterns, the patterns also scatter the light in a similar manner and it is often impossible to distinguish scattering due to particles from that due to the pattern. Accordingly, these devices are limited to inspection of foreign particles on patternless wafers.
In U.S. Pat. No. 4,441,124, Heebner uses a plurality of photodetectors in a ring system to monitor the intensity of light scattered substantially along the wafer surface. Since a patterned wafer with no particulate matter thereon will scatter substantially no light along the wafer surface, while a wafer having particulate matter thereon will scatter a portion of the light impinging thereon along the surface, the apparatus can be used to image particles contaminating the wafer surface without imaging the pattern.
In U.S. Pat. No. 4,342,515, Akiba et al. discloses a method and apparatus using polarized light for detecting particles on a surface, as for example, the surface of a semiconductor wafer. A laser beam with s-axial polarization is projected sidewards towards the wafer to be inspected. The light reflected from the wall of a bump conforming with an etched pattern area remains polarized in the s-axial direction so is intercepted by a polarizer plate. Light reflected from foreign matter is depolarized, so as to generate light waves in both the s-axial and p-axial directions, so that the p-axial polarization component passes through the polarizer plate and is detected by a transducer.
In a dual beam particle detection system, two laser beams are directed at equivalent positions on two presumably identical die. The system has two detectors, one for each beam which detects light scattered by each die. The intensities are compared, and any difference is assumed to be due to a particle. Such a system requires precise positioning to ensure that the equivalent spots on the two die are being simultaneously observed. Unfortunately, due to nonlinearities in scanning die at differing positions, the scanning control required is quite complex.
In U.S. Pat. No. 4,579,455, Levy et al. disclose a method and inspection apparatus for detecting defects in reticle or photomask having multiple patterns thereon. An illuminator illuminates a photomask to be inspected, while left and right inspection optics project images of two duplicate die patterns of the photomask onto a one-dimensional multielement detector. A stage moves the photomask at a constant velocity in a direction normal to the length of the detector to allow the detector to sequentially view the entire area to be inspected. Digitizers convert the analog signals of the detector into values which are stored temporarily in two pixel memories. To minimize the memory requirements, the pixel memories hold pixel values for only a small number of scans. As the pixel values are shifted through the pixel memories, a defect detector circuit analyzes groups of pixels representing corresponding areas of the two die patterns. Defects are detected in 3-by-3 comparison matrices of pixels by a process of area subtraction involving the calculation of error values by summing the squares of the differences of each of the 9 pixel values in the comparison matrices and in 24 adjacent matrices. If none of the 25 resulting error values is less than a threshold, a defect is assumed. Thus, this method is essentially a spatial test of pixel by pixel comparison with some allowance for misalignment built in.
It is an object of the present invention to provide a relatively fast method and apparatus which is capable of detecting particles with sizes on the order of one micron or smaller on patterned wafers, photomasks and the like.
It is another object of the present invention to provide a particle detection method and apparatus for patterned wafers, photomasks and the like, which optimizes the ratio of particle to pattern detection, which does not require the extremely accurate tracking of dual beam systems, and which reduces or eliminates problems due to nonlinearities, defocusing and scan misalignments.